Tiny organisms remember the way to food

SOME of the most basic organisms are smarter than we thought. Rather than moving about randomly, amoebas and plankton employ sophisticated strategies to look for food and might travel in a way that optimises their foraging.

Biophysicists have long tried to explain how creatures of all sizes search for food. However, single-celled organisms such as bacteria seem to move in no particular direction in their search.

To investigate, Liang Li and Edward Cox at Princeton University studied the movements of amoebas (Dictyostelium) in a Petri dish, recording the paths travelled by 12 amoebas, including every turn and movement straight ahead, for 8 to 10 hours per amoeba.

Immediately after an amoeba turned right, it was twice as likely to turn left as right again, and vice versa, they told a meeting of the American Physical Society meeting in Denver, Colorado, last week. This suggests that the cells have a rudimentary memory, being able to remember the last direction they had just turned in, says Robert Austin, a biophysicist at Princeton who was not involved in the study.

Such memories might be laid down because of the way the cell moves. To turn, an amoeba extends part of its body in the preferred direction, which creates a scar made of protein down that side of the cell. The scar might make the cell temporarily more likely to move in the opposite direction. For the amoeba, the pay-off is that it avoids travelling in circles and hence can search a larger area.

Another study has shed light on the optimal foraging strategy of micro-organisms. Ricardo Garcia and Frank Moss at the University of Missouri at St Louis placed Daphnia zooplankton (see Picture) in water laced with food particles and measured the angles of up to 1000 turns made by individuals of five different species. The plankton were exponentially more likely to turn at smaller, more gradual angles than larger ones, with a preferred turning angle of about 36 degrees.

Meanwhile, their collaborators at Humboldt University in Berlin, Germany, used a computer model to calculate what angle would maximise the amount of food Daphnia collected within a fixed space and time. It turns out that the optimal turning angle closely matches that used by Daphnia in the experiments (Mathematical Biosciences, in press); and an angle any greater or smaller would make the plankton less successful. "This allows it to explore the most area and get the most food in a given amount of time," says Garcia.

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